In a radio communication network including a plurality of cells, a load is placed on each cell from traffic. The cells that receive higher loads have higher probabilities to experience call loss and packet discard. Accordingly, in designing such a radio communication network, it is preferable that a traffic load is appropriately set up for the traffic capacity of each cell. Further in a radio communication network including a plurality of cells, there are spots where the communication quality degrades due to interference between transmission signals from different cells. Accordingly, it is preferable to reduce such spots where communication quality degrades in a predetermined area. Here, the ratio of the spots where communication quality degrades and which occupy the predetermined area will be named the degradation ratio; for example, a spot where SIR (signal power-to-interference power ratio) does not satisfy the predetermined value will be named a spot with degraded communication quality.
For example, if all the cells have an equal traffic capacity and the requirement for degradation ratio is uniform across the whole area, it is preferable that all the cells undergo equal traffic load in view of load balancing. Conventionally, there is known a technique for controlling the load on each cell in a radio network (see Japanese Patent Application Laid-open 2005-117357).
The traffic load on each cell is equal to a product of “the traffic quantity expected to arise per unit area (expected traffic quantity per unit area)” and “the cell area”. Accordingly, if, for example the expected traffic quantity per unit area is uniformly distributed, it is preferable that radio network design be carried out so that all the cells will be equal in area size.
In radio network designing, normalization of the traffic loads on all the cells will be called cell-balancing. Specifically, this corresponds to the case where the traffic load on each cell is adapted to be close to the others if the demanded degradation ratio is equal across the whole area, or the case where each cell has an area whose size is close to the others when the expected traffic quantity per unit area is uniformly distributed.
The traffic load on each cell is usually determined by a simulation using a radio network designing tool. Specifically, the radio network designing tool determines first the cell boundary of every cell from the tilt angles of the antennas, transmission powers from the antennas, terrain undulations, mutual interference at each cell and the like, then calculates the cell areas. Next, the radio network designing tool multiplies the expected traffic quantity per unit area by the cell area so as to calculate the traffic load on each cell.
In the conventional radio network designing tool, the operator of the radio network designing tool modifies the parameters such as tilt angles of antennas and transmission powers and repeats calculating processes of traffic loads to thereby determine proper parameters that can strike a balance between cells.
Incidentally, in radio network designing there are some cases in which the degradation ratios in important areas are demanded to be suppressed to be lower than other areas. In answer to this, there has been proposed a method of automatically setting up parameters to reduce the degradation ratios in important areas by using a radio network designing tool.
FIG. 1 is a chart showing one example of the distribution of important areas. In FIG. 1, in whole area 90 enclosed by the thick solid line a plurality of cells 911˜916 are formed. Cell 911 is assumed to have cell number j=1, cell 912 cell number j=2, cell 913 cell number j=3, cell 914 cell number j=4, cell 915 cell number j=5 and cell 916 cell number j=6. The hatched areas are important areas 92. Each base station 93 can constitute three sectors, and in FIG. 1 the relationship between the sector of each base station 93 and its covering cell is shown by an arrow.
For example, a case will be considered where the degradation ratios of important areas are suppressed to be lower than other areas by using the tilt angles of antennas as parameters. Important areas 92 exist in cell 911 with cell number j=1 and in cell 916 with cell number j=6. Accordingly, the tilt angles of the antenna for cell 911 and the antenna for cell 916 are modified to be greater values in order to lower the degradation ratios of cell 911 and cell 916. Here, it is assumed that as the tilt angle is made greater, the cell area becomes smaller, so that the traffic load is reduced.